Bones change rotation to first frame of last animation
Make sure the action isn't still associated with the object, either via the action editor or NLA.  To see if an object has an animation look at the outliner

 

Here the Cube has an action (CubeAction.001) a driver and has NLA tracks.  The camera has no animation.  

How to clear all animations. 

Select all the bones (pose mode)  and  clear transform


You can make a "zero" animation action here by inserting a LocRotScale.